This invention is directed to a process for preparing polyarylates of improved color and having a reduced viscosity of from 0.5 to greater than 1.0 dl/g, which process comprises reacting the diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent, based on the weight of the polyarylate produced, of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260.degree. to about 350.degree. C. Optionally, the process may be carried out in the presence of a magnesium, manganese, or zinc catalyst.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane (also identified as Bisphenol-A), and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative which is then reacted with an aromatic dicarboxylic acid to form the polyarylate. However, heretofore, the diacetate process has been carried out by several different methods as illustrated in the following U.S. patents:
In U.S. Pat. No. 2,595,343 issued May 6, 1952, an aromatic bisphenol is first reacted with acetic anhydride to form bisphenol diacetate which is then condensed with an aliphatic dicarboxylic acid to form a polyarylate by an acid interchange reaction. The condensation reaction is performed in the presence of an acid catalyst, such as para-toluenesulfonic acid, phosphoric acid or sulfuric acid. However, this acid exchange reaction does not yield an acceptable product when an aromatic dicarboxylic acid is substituted for the aliphatic acid.
U.S. Pat. No. 3,225,003 issued Dec. 21, 1965, describes the preparation of novel copolyesters by reacting hydroquinone diacetate with a mixture of hexahydroterephthalic acid and hexahydroisophthalic acid in the presence of a catalyst, such as sodium acetate, at a temperature of 230.degree. C., and in an organic solvent such as 1, 1,2-trichloroethane. The reaction is exemplified as being carried out in the presence of anhydrous sodium acetate catalyst and eutectic mixtures of biphenyl and diphenyl oxide (diphenyl ether) at 230.degree. C. in three stages for a total reaction time of about 28 hours. Thus, this process requires long reaction times and several steps.
U.S. Pat. No. 3,317,464, issued May 2, 1967, describes the preparation of linear aromatic polyesters by the polycondensation of diphenols, or their diacetates, with polynuclear aromatic dicarboxylic acids. The examples of this patent describe the preparation of the polyester using catalysts, such as mixtures of p-toluene sulphonic acid and antimony trioxide, or butyl orthotitanate, optionally in acetic acid. The reaction is carried out in a molten mixture of reagents and catalysts by heating these together under sub-atmospheric pressure. The patent states, but does not exemplify, that the reaction may be carried out in solution, in inert solvents, such as alpha-methyl-naphthalene, biphenyl or diphenyl oxide.
U.S. Pat. No. 3,329,653 issued July 4, 1967 describes the preparation of high molecular weight linear condensation polymers, such as aromatic polyesters, aliphatic polyamides, and polycarbamides. These polymers are formed at or below the melting point of the polymer by carrying out the reaction while the reacting materials are suspended in an inert non-solvent medium, with a swelling agent for the condensation polymer which also needs to be present in the reaction medium. Example 7 of this patent describes the preparation of poly[2,2-bis(4-hydroxyphenyl)propane isophthalate] by heating a mixture of 780.9 g of the diacetate of Bisphenol-A, 415.3 g of isophthalic acid, 900 g. of Apco Inkol No. 0, 25 g of sulfolane swelling agent and 2.5 g of sodium methoxide catalyst. The reaction is held at reflux for 40 hours. The patent describes, as particularly effective swelling agents, sulfolane, diphenyl ether, and quinoline. Thus, this process requires the use of large quantities of a non-solvent, swelling agents as well as long reaction times.
U.S. Pat. No. 3,824,213 issued July 16, 1974, describes the preparation of halogenated aromatic polyesters by reacting in solution, an aliphatic carboxylic ester of a halogenated bisphenol, such as tetrachlorobisphenol-A with an aromatic acid mixture of terephthalic and isophthalic acids at a temperature of 220.degree.-350.degree. C., in the presence of a catalytically effective amount of a cobalt, nickel, or manganese salt of an aliphatic carboxylic acid. This patent describes that the esterification reaction may be conducted with said catalysts in a suitable solvent, such as a hydrocarbon, halogenated aliphatic or aromatic hydrocarbon or the like (i.e. a solvent which is inert under the reaction conditions employed). The solvents which are specifically mentioned in the patent are: diphenyl ether, benzophenone, dichloroethane and dichlorobenzene. This patent exemplifies that several prior art catalysts, i.e., antimony oxide, zinc chloride, magnesium, magnesium oxide, magnesium acetate, calcium acetate, triphenyl phosphine, sulfuric acid, phosphoric acid, hydrochloric acid, borontrifluoride, and trifluoromethyl sulfonic acid all yield unacceptable polymers, in that the polymers have low I.V. values. The particular cobalt, nickel or manganese salts, as described in this patent, are necessary to yield polyesters having an inherent viscosity of at least about 0.2, which is considered an acceptable viscosity in that patent.
U.S. Pat. No. 3,948,856, issued Apr. 6, 1976, describes an acid interchange polymerization process for producing an aromatic polyester by reacting substantially stoichiometric amounts of an aromatic diester with a dicarboxylic acid at a temperature of 220.degree.-350.degree. C., in a solvent, and in the presence of a catalyst, which is a mixture of a transition metal salt of a strong inorganic acid and a transition metal salt of an aliphatic carboxylic acid. The aromatic diester and aromatic dicarboxylic acid are reacted in the presence of a solvent which must have a boiling point above about 220.degree. C. The solvents specified are: diphenyl ether, halogenated diphenyl ether, diphenyl sulfone, benzophenone, polyphenyl ethers such as tri and tetraphenyl ether, terphenyl, tetraphenyl and halogenated biphenyl.
U.S. Pat. Nos. 3,684,766, issued Aug. 15, 1972, and 3,780,148, issued Dec. 18, 1973, describe a variation of the diacetate process. In the patented processes, a prepolymer is formed from, for example, a diacetate, such as Bisphenol-A diacetate, and an aromatic acid, in the presence of a catalyst. The prepolymer so formed is then comminuted into small particles. These particles are then contacted with a crystallizing agent to crystallize the polyester. The crystallized polyester is heated in the presence of an inert gas and under reduced pressure to increase the inherent viscosity of the polyester. However, the processes described in these patents require multi-steps including the step of crystallizing the prepolymer.
U.S. Pat. No. 4,075,173 issued Feb. 21, 1978, describes the preparation of copolyesters by reacting an aromatic dicarboxylic acid, a diacetate of Bisphenol-A, and an acetate of p-hydroxybenzoic acid. Various processes for producing polyarylates by the reaction of Bisphenol-A and terephthalic and isophthalic acids are reviewed in this patent. The following process for producing polyarylates, identified as route (1), is described in column 2, of the patent: ##STR1##
This process is the diacetate process as described herein, or the "Acetate process" as defined in the patent.
Column 2 of the patent states:
"The route (1) is not desirable because the undesirable coloration and deterioration of polymer are particularly remarkable as disclosed in the above-mentioned literature." PA1 "On the other hand, the route (1), Acetate process, is economically advantageous because the materials used are cheap and the operation is simple. For example, diacetate of bisphenol-A, a monomer for Acetate process, is synthesized by merely reacting acetic anhydride and bisphenol-A. Consequently, it may be said that, if the fatal drawbacks of Acetate process, coloration and deterioration, are solved, Acetate process will become the most superior process."
Further, column 3 of the patent states:
Thus, the skilled workers in the field of polyarylate chemistry realize that the existing processes for producing polyarylates have one or more deficiencies, including a color problem, and that a need exists to develop a viable diacetate process for producing polyarylates wherein coloration is improved.
In U.S. Pat. No. 4,075,173, a copolyester was prepared by the diacetate process by a solid-state polymerization of low molecular weight polymers without using crystallizing agents. The patentees state that their Acetate process is possible only when specific monomers are combined to form the prepolymer. These monomers are the diacetates of bisphenol-A, terephthalic acid and/or isophthalic acid and an acetate of p-hydroxybenzoic acid. The prepolymer is then converted to the desired high molecular weight polymer by solid state polymerization.
Thus, the diacetate processes for producing polyarylates by the procedures of the aforediscussed U.S. patents, and as stated in U.S. Pat. No. 4,075,173, are generally unsuitable since they are either economically unattractive (they have poor color) and/or produce a commercially unmarketable product.
Therefore, a need exists for an economical and practical diacetate process for producing high molecular weight polyarylates having improved color.
In the diacetate process for producing polyarylates, problems exist which must be additionally solved in order to have a viable, economically attractive process. One problem is that the production of polyarylates by the diacetate process using existing catalysts, solvents, techniques, etc. produces polyarylates which are colored. Thus, these polyarylates have limited uses, since they cannot be used to produce molded products where good color is required.
Another problem when a diester derivative of a dihydric phenol is reacted with an aromatic dicarboxylic acid in the molten state is that sublimation of the diacid occurs. This disrupts the stoichiometry of the reaction and the polyarylate produced is not of acceptable molecular weight. To prevent sublimation of the diacid, several techniques have been developed. These include the use of large amounts of solvents together with a variety of catalysts and generally long reaction times in the polymerization process. However, these techniques are quite costly and do not provide an optimum process. Another problem when a diester derivative of a dihydric phenol is reacted with an aromatic dicarboxylic acid in the molten state is that the viscosity of the system increases dramatically towards the end of the reaction and therefore the reaction becomes diffusion controlled (the molecules are not close enough to insure rapid reaction) rather than kinetically controlled. Also, the polymer product is difficult to handle (i.e., removal from the reactor) due to this high viscosity.
Yet another problem in the production of polyarylates by the diacetate process is that a carboxylic acid is a by-product of the reaction of a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid. In order to provide an efficient economical process and a high molecular weight polyarylate, the acid, for example, the acetic acid has to be conveniently and efficiently removed.
It has now been discovered that a polyarylate having improved color and having a reduced viscosity of at least about 0.5 to greater than 1.0 dl/gm can be efficiently and economically produced. The present process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent, based on the weight of the polyarylate produced, of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain a benzylic and/or tertiary hydrogen atom, at a temperature of from about 260.degree. to about 350.degree. C. Optionally, the process may be carried out in the presence of a catalyst.
The utilization of between about 10 to about 60 weight percent of at least one of the cycloaliphatic, substituted aromatic, or heteroaromatic compounds and optionally, a catalyst, as will hereinafter be described, in the diacetate process, results in the production of a polyarylate which has improved color, and thus, the polyarylate can be used to produce molded articles of improved clarity. Also, the utilization of said compounds, in the diacetate process, prevents sublimation of the aromatic dicarboxylic acid; thus producing polyarylates of acceptable molecular weight and also provides for better removal of the acetic acid by-product. Further, an additional benefit in using the alkyl substituted aromatic, heteroaromatic or cycloaliphatic compounds in the amounts indicated is that the viscosity of the system is decreased. This decrease in viscosity provides a faster reaction time since better mixing of the reactants occurs which allows the reaction to proceed under kinetic control. Additionally, by using these compounds, reaction times are relatively short so that a polyarylate is produced generally in less than 10 hours at the reaction temperatures. Furthermore, the present process can be carried out at atmospheric pressure or supra atmospheric pressures and therefore avoids the use of the costly equipment which is needed by the prior art processes which carry out the diacetate processes under vacuum.